From Bulbapedia, the community-driven Pokémon encyclopedia.
A Youngster (Japanese: たんパンこぞう Shorts Boy) is a type of Trainer introduced in Generation I games. They have been depicted as young boys wearing caps who are often obsessed with wearing shorts.
In the games
Youngsters are among the first opponent Pokémon Trainers players will find in the games, with Youngsters abounding on the first few routes of a region. They are known for typically staying close to home, and will most often have on their teams the Pokémon available on early routes or their evolved forms.
Red, Green, Blue, and Yellow
...and so on...